The present invention generally relates to power amplifiers, and more particularly relates to methods and apparatus for compensating an input signal for distortion introduced to the input signal by a power amplifier.
The design of radio-frequency power amplifiers for communications applications often involves a trade-off between linearity and efficiency. Power amplifiers are typically most efficient when operated at or near a so-called “saturation point.” However, the response of the amplifier at or near the point of saturation is non-linear. Generally speaking, when operating in the high-efficiency range, a power amplifier's response exhibits a nonlinear response and memory effects of varying duration.
One way to improve a power amplifier's efficiency and its overall linearity is to utilize a predistorter to digitally predistort the input to the power amplifier to compensate for the distortion introduced by the power amplifier. In effect, the input signal is adjusted in anticipation of the distortion to be introduced by the power amplifier, so that the output signal is largely free of distortion products.
Generally, the predistortion is applied to the signal digitally, at baseband frequencies, (i.e., before the signal is upconverted to radio frequencies). Predistortion techniques can be quite beneficial in improving the overall performance of a transmitter system, in terms of both linearity and efficiency. However, recent advances in power amplifier technology have yielded power amplifiers exhibiting more sophisticated and complex characteristics that current predistortion models are inadequate to handle.